1. Field of the Invention
This invention relates generally to a luminance signal forming circuit and, more particularly, is directed to a luminance signal forming circuit for use with a color television camera having complementary color filters.
2. Description of the Prior Art
As is well known, complementary color filters may be used to block a selected color component of the primary colors, that is, red (R), green (G) and blue (B). A yellow (Y.sub.E) complementary filter passes R and G components therethrough, a cyan (C.sub.Y) complementary filter passes G and B components therethrough, a magenta (M.sub.G) complementary filter passes R and B components therethrough, and a white (W) complementary filter passes all of the R, G and B primary color components therethrough.
Thus, two or more primary color components, for example, color components R and G, color components G and B, color components R and B, or color components R, G and B, can pass through a single filter when the described complementary color filters are used in a color television camera. As a result the number of filters in the light path can be reduced so that there is an increase in the quantity of light of the primary color components incident on the image pick-up device, such as, the image pick-up tube or solid state image pick-up element. Accordingly, even when an image of a relatively dark object is projected onto the image pick-up device, a video signal output having a relatively high level can be obtained from the camera using complementary filters.
As disclosed in U.S. patent application Ser. No. 07/100,223, filed Sept. 23, 1987, and having a common assignee herewith, and which was allowed Sept. 28, 1988, it has been proposed to provide a color television camera having complementary color filters with a luminance signal forming circuit in which complementary color signals are separated from the output signals of the television camera, primary color signals are obtained from the complementary color signals and, in a matrixing operation, a first luminance signal and suitable color-difference signals, such as, R-Y and B-Y, are obtained from the primary color signals. A second luminance signal is obtained merely by mixing the complementary color signals, and the resulting first and second luminance signals are combined for providing a composite luminance output signal. In the foregoing proposed luminance signal forming circuit, the first luminance signal obtained in a matrixing operation from the primary color signals has good luminance reproducibility or fidelity, but a relatively poor S/N ratio due to color noise. The second luminance signal obtained by merely mixing the complementary color signals is, in fact, a pseudo luminance signal in that the ratios of the primary color signal components therein deviate from the standard, so that the second luminance signal has relatively poor luminance reproducibility or fidelity, but a relatively high S/N ratio. In forming the composite luminance output signal, it is intended that the good luminance reproducibility of the first luminance signal will be obtained while maintaining the high S/N ratio of the second luminance signal. However, in designing the proposed luminance signal forming circuit, a compromise has to be made between the attainment of good luminance reproducibility and a high S/N ratio, and both of these characteristics can not be appropriately maintained for all signal states, that is, when the output signals from the television camera have high levels and low levels, respectively.